Porous materials comprising a polytetrafluoroethylene (hereinafter abbreviated as "PTFE") as a material are used in a wide variety of fields such as separator for cells, membrane filters, electric wires, analytical instruments and artificial blood vessels. Now, in recent years, porous materials of PTFE having a minute pore diameter and excellent permeability have been required of application fields such as precision filters, high-performance separation membranes and artificial lung septa. Therefore, porous materials of PTFE having micropores and a high porosity have been required. However, it has been very difficult to produce a porous material of PTFE, which combines micropores with a high porosity and has excellent permeability.
As a process for producing a porous material of PTFE, it has heretofore been known to stretch an unsintered molded article obtained by paste extrusion of PTFE at a temperature not higher than the melting point of PTFE and then sinter the molded article (Japanese Patent Publication No. 13560/1967). According to this process of stretching the unsintered molded article, porous material of PTFE having various porosities can be obtained. However, the pore diameter becomes greater as the draw ratio is increased to enhance the porosity. Therefore, there has been a limit in the production of porous material of PTFE combining micropores and a high porosity.
As another process for producing a porous material of PTFE, it has also been proposed to heat a molded article of PTFE to a temperature not lower than 327.degree. C., slowly cool the sintered molded article so as to heat-treat it to give a crystallinity of 80%, and then uniaxially stretch the sintered molded article at a draw ratio of 1.5-4 times at a temperature of 25-260.degree. C. (Japanese Patent Publication No. 42794/1978). According to this process (hereinafter abbreviated as "the slow cooling process"), a porous material of PTFE in which micropores have been formed can be obtained. In the slow cooling process, however, crystallization is not allowed to fully progress if the cooling rate is too fast in the step of slowly cooling the sintered molded article of PTFE. It is thus necessary to decelerate the cooling rate. Accordingly, this process has involved a problem that precise temperature control and large equipment are required.
More specifically, it is said in the slow cooling process that it is preferable to cool the sintered molded article generally at a rate slower than about 0.5.degree. C./min for enhancing the crystallinity of the resulting sintered molded article of PTFE. In Examples of this publication, cooling rates of 0.25.degree. C./min, 0.1.degree. C./min and 0.05.degree. C./min are used. In order to perform slow cooling at such a low cooling rate, it is necessary to conduct temperature control with extremely high precision. In addition, porous materials of PTFE are generally formed as continuous molded articles such as rods, tubes, strips and sheets by paste extrusion of fine powder of PTFE. The PTFE porous material is formed through a heat-treating step, a stretching step and the like. It is however difficult and impracticable to apply the slow cooling process to these continuous sintered molded articles. For example, in order to cool a sintered molded article in the form of a continuous sheet from 350.degree. C. to 290.degree. C. at a cooling rate of 0.5.degree. C./min by means of an oven 3 m long, it is necessary to pass the sheet through the oven over 2 hours. The transit time in the oven is 1.5 m/hr in terms of linear velocity. Therefore, in the case where the length of the continuous sheet is 100 m, it takes about 67 hours to pass through the oven. On the other hand, in order to slowly cool a continuous sheet 100 m long in 20 hours under the above-described cooling conditions, it is necessary to pass the sheet through the oven at a linear velocity of 5 m/hr. Therefore, a large oven as long as 10 m is required.
As described above, in the process in which the sintered molded article is slowly cooled from the temperature not lower than the melting point of PTFE, the production of a continuous sintered molded article requires either a very long oven or a treatment at a very low linear velocity. Therefore, there is a limit in industrial practice.
Japanese Patent Application Laid-Open No. 78823/1989 discloses a production process of a porous PTFE membrane, in which fine powder of a PTFE having a number average molecular weight of 1,000,000 or lower is paste-extruded into a molded article, the molded article is sintered, the sintered molded article is slowly cooled from the sintering temperature at a rate lower than 10.degree. C./hr (1.degree. C./hr in Example 1) to enhance its crystallinity, and the thus-cooled sintered molded article is then stretched in at least an uniaxial direction. One of the co-inventors of the present invention proposed a process in which a continuous molded article of PTFE is sintered, and the resultant sintered molded article is slowly cooled while passing the sintered molded article through at least two different zones, which are successively preset from a higher-temperature region to a lower-temperature region in a temperature range of 350-290.degree. C. and are controlled at substantially fixed temperatures, thereby enhancing its crystallinity, and previously applied for a patent (Japanese patent Application Laid-Open No. 8344/1994). When the molded articles enhanced in crystallinity according to these processes are stretched, porous PTFE membranes having micropores and a high porosity can be obtained. However, the molded articles enhanced in crystallinity according to these processes tend to break if they are stretched at a draw ratio of 10 times or higher. As a result, the porosities of the resulting porous PTFE membranes have been at most 65% or so. The reason for it is considered to be attributable to the fact that in these processes for enhancing crystallinity, the molded article is held for a considerably long period of time at a temperature not lower than the melting point of PTFE, and so microstructural thermal decomposition occurs, thereby reducing elongation percentage.
A PTFE filter is excellent in heat resistance and chemical resistance and hence used mainly in filtration of chemicals and gases in a field of semiconductors. With the high integration of semiconductors in the field of semiconductors, there is a strong demand for development of a PTFE filter having a minuter pore diameter. Since the yield of high-integrated semiconductors is affected by the retention of a PTFE filter, there is a demand for development of a filter high in the retention of fine particles. Namely, judging from the recent performance requirement for PTFE filters, it is desirable that the 0.109 .mu.m latex retention be at least 90%, preferably at least 99%, more preferably 100%. In commercially-available PTFE filters (pore diameter: 0.1 .mu.m and 0.05 .mu.m), however, the 0.109 .mu.m latex retention is up to a maximum of about 70%. On the other hand, it has been known a porous PTFE membrane having a pore diameter of 0.02 .mu.m. However, its flow rate (IPA flow rate) as determined with isopropyl alcohol is as extremely low as 0.0005 ml/cm.sup.2 /min (as measured under a differential pressure of 0.95 kg/cm.sup.2), and so the filter is lacking in practical performance.